1. Field of the Invention
The invention relates to a nonaqueous electrolyte secondary battery and to a method of producing the same. More particularly, the invention relates to a sealed nonaqueous electrolyte secondary battery that is provided with a current interrupt device that operates by a rise in internal pressure and also relates to a method of producing this battery.
2. Description of Related Art
Lithium ion secondary batteries and other secondary batteries are smaller, lighter, and have higher energy densities than older batteries and exhibit better output densities. As a consequence, they have in recent years entered into preferential use as so-called portable power sources for, e.g., personal computers and portable terminals, and as vehicle drive power sources.
One embodiment of these batteries is the sealed nonaqueous electrolyte secondary battery. This battery is typically fabricated by inserting an electrode assembly containing positive and negative electrodes into a battery case along with a nonaqueous electrolyte (typically a nonaqueous electrolyte solution) and then applying a lid and closing off (sealing) the opening. The positive electrode in a sealed nonaqueous electrolyte secondary battery has a structure in which an electrode material—which is composed mainly of a material (the positive electrode active material) that can reversibly engage in the insertion and extraction of the chemical species (for example, the lithium ion) that constitutes the charge carrier—is formed as a layer on an electroconductive member (the positive electrode current collector); this layer-formed material is referred to below as the “positive electrode mixture layer”. One example of this positive electrode active material is a complex oxide that contains at least nickel, cobalt, and manganese as transition metal elements (for example, a lithium complex oxide). Such a complex oxide is a positive electrode active material that exhibits a high capacity and an excellent thermal stability.
The electroconductivity of the positive electrode may be reduced (the battery resistance may be increased) depending on the positive electrode active material present in the positive electrode mixture layer. The electroconductivity of the positive electrode undergoes a decline when a positive electrode active material is used that itself has a low electroconductivity, such as complex oxides that contain at least nickel, cobalt, and manganese as transition metal elements (“NCM complex oxides” below). When the proportion of the conductive agent present in the positive electrode mixture layer is raised in order to raise the electroconductivity of the positive electrode, the battery capacity is then impaired due to the decline in the proportion of the positive electrode active material. As a consequence, pressing the positive electrode mixture layer at high pressures (raising the density of the positive electrode mixture) during formation of the positive electrode mixture layer can be considered for the purpose of raising the electroconductivity of the positive electrode. This improves the electroconductivity of the positive electrode by raising the intimacy of contact (increasing the contact area) between the positive electrode active material and the conductive agent.
Sealed nonaqueous electrolyte secondary batteries are generally used under controlled conditions in which the voltage resides in a prescribed range (for example, from 3.0 V to not more than 4.2 V); however, overcharging can occur when the prescribed voltage is exceeded when a greater than normal current is supplied to the battery. Current interrupt devices (CIDs)—which interrupt the charging current when the voltage in the battery case reaches or exceeds a prescribed value—are widely used as an overcharge countermeasure. When the battery assumes an overcharged state, for example, the nonaqueous solvent in the nonaqueous electrolyte typically undergoes electrolysis with the generation of a gas. The current interrupt device cuts the charging circuit for the battery based on the gas generation and can thereby prevent further overcharging. With regard to the use of this current interrupt device, a method is available in which a compound having a lower oxidation potential (i.e., a lower voltage at which oxidative decomposition begins) than the nonaqueous solvent in the nonaqueous electrolyte is incorporated in the nonaqueous electrolyte in advance (such a compound is referred to below as a “gas generator”). When the battery enters an overcharged state, the gas generator rapidly undergoes oxidative decomposition at the surface of the positive electrode with the production of the hydrogen ion (H+). The hydrogen ion diffuses in the nonaqueous electrolyte and is reduced on the negative electrode to produce hydrogen gas (H2). The current interrupt device can be rapidly operated due to the rise in pressure within the battery case brought about by this gas generation.
When, as noted above, the positive electrode mixture layer is subjected to high-pressure pressing (raising the density of the positive electrode mixture) in order to raise the electroconductivity of a positive electrode that contains a relatively weakly electroconductive NCM complex oxide, the pores (voids) in the positive electrode mixture layer are reduced (in particular there is a tendency for the relatively large pores to be diminished). Since a film originating from the gas generator may block the pores in the positive electrode mixture layer when the gas generator undergoes decomposition, the failure to achieve a satisfactory pore formation in the positive electrode mixture layer can result in a decline in the amount of gas that can be generated within the battery case during a pressure rise within the battery case and the current interrupt device may then be unable to undergo rapid operation.